'She has an extraordinary understanding of what is going on musically'

Despite comparisons with Callas, the 50-year-old mezzo-soprano from California is relatively little known in the UK. But with her appearance at the Proms tonight, she may finally be about to reach a wider British audience

The Proms tend to start with a flourish. There will be some grand-scale work, one that will take advantage of the immensity of the Royal Albert Hall. And then there will be a star. In recent years, we've seen the flashy talents of Evgeny Kissin, Maxim Vengerov and Lang Lang. Tonight, however, the slot will be occupied by a presence of a very different order: Lorraine Hunt Lieberson.

For a woman who has been called by the New Yorker "a mezzo with the most potent voice since Callas", she resonates remarkably lightly on the popular consciousness. It's partly that this 50-year-old Californian has worked relatively little in the UK, and partly that she started her career as a viola player. And it is partly that she has remained consistently uninterested in fame, or even in maintaining a sustained relationship with a recording label.

Yet ask those in the know and the superlatives come pouring out. According to Andrew Clements, the Guardian's chief music critic, "She's the best mezzo-soprano since Janet Baker. She's miles better than any of the others working now ... The only two other singers in the same class as her are [Finnish soprano] Karita Mattila and [Welsh baritone] Bryn Terfel."

It is her directness - what the Proms controller Nicholas Kenyon calls the "piercingly personal qualities she brings to everything she sings" - that grasps audiences. To hear her at Glyndebourne last year, where she sang Irene in Peter Sellars' production of Handel's Theodora, was to witness her scooping out immensities of emotion from some deep part of herself, such that the experience of watching her felt almost dangerous in its intensity. She exposes herself utterly; there seems to be no mediating barrier, no artifice.

David Pickard, who runs Glyndebourne, says: "I don't think there's any performer who has the emotional truth and directness of Lorraine, who has her ability to speak to everyone in the audience. How does she do it? However much one tries to analyse it, it's almost impossible to do so. At Glyndebourne last year, people were coming away in tears after her performances. And then in September I went to hear her sing Britten's Phaedra and Jocasta in Stravinsky's Oedipus Rex at the Proms. There, there was none of the intimacy of the opera house at Glyndebourne, no production, no costumes. I couldn't believe she'd be capable of having the same powerful effect: but she did."

Hunt Lieberson is slender with a cloud of honey-mousy hair, a slightly homespun look and a straight-backed, elegant poise that you often see in dancers.

She was raised in the San Francisco Bay area, one of four children. Her father was a music teacher who conducted local community operas, her mother a singer. Her father was ambitious for his daughter: she recalls afternoons being made to listen to the Metropolitan Opera Saturday broadcasts, often with a score on her lap.

Her first instrument was the piano, then the violin and ultimately the viola. And it was a viola player that she thought she was destined to be.

After studying at San Jose State University (viola and voice, with the emphasis on the latter) she had a boyfriend who was arrested in Mexico for buying marijuana. They bribed the guards to let her live with him in his low-security jail. "It really was high adventure," she said. "I always think my fantasy role would be Leonore in Fidelio - I would be absolutely perfect for it." (Leonore is Beethoven's heroine who disguises herself as a man and infiltrates a prison to free her husband.)

A later boyfriend was a French horn player who landed a job in Boston. Hunt Lieberson moved east to be with him. It was here that singing finally took over, and the key artistic relationship of her career - with the theatre and opera director Peter Sellars - was forged.

She recalled: "I was singing in a professional choir, and they were going to do Handel's Julius Caesar. I was asked to audition for the part of Sesto [Caesar's son]. This was 1985 ... It was my first big gig, and it was pretty transforming. When I first did it I was quite wild, quite fiery on stage. I was playing a 14-year-old boy, running around with guns, avenging my father's death.

"At first it was completely overwhelming to do all this highly choreographed movement, and also to sing well. I really went for it. Later I heard a tape, and it was like ... I was all over the place, but it was a great opportunity to go to the edge, and actually go too far maybe, where most stage directors I know wouldn't have gone, or would have been reining me in."

In 1988 her fate was sealed by the simple fact that her viola - uninsured - was stolen. A decade of formidable work as a singer followed. In 1996 she sang Irene in Theodora at Glyndebourne for the first time. It was, according to Andrew Clements, "a complete revelation", not least because the Handel oratorio was so rarely heard at the time.

Marshall Marcus, a founding member and now chief executive of the Orchestra of the Age of the Enlightenment, the orchestra that played for the Theodora performances, believes that Hunt Lieberson's early incarnation as a viola player is germane. "You are aware of someone who has an extraordinarily multifaceted understanding of what is going on musically. You sense that the way she works is to start with the demands of the music, rather than with an idea of what she can do technically."

In 1997 she met the man who two years later would become her husband, the composer Peter Lieberson. It was a marriage made in opera: he wrote a part for her in his work Ashoka's Dream. "I was playing Ashoka's second wife, and I became Peter's second wife," she said.

But there was also personal turbulence. Her younger sister Alexis died from breast cancer in May 2000; two months before, Hunt Lieberson was also diagnosed with the disease.

The previous year she had begun working with Sellars to stage a pair of Bach cantatas, Ich habe genug (It is enough) and Mein Herze schwimmt im Blut (My heart is bathed in blood).

The dramatic imagery was inescapably concerned with terminal disease and death - "there were tubes hanging out of me", as Hunt Lieberson put it. The next year, following her double lumpectomy, they picked up the project again, and it was eventually seen at the Barbican in London.

For some, the staging was intolerably pretentious, but few doubted Hunt Lieberson's conviction or the power of her performance. Unsurprisingly, it was a deeply personal experience: "It didn't make me plunge into grief ... my sister had been sick for a few years and I had been grieving all along, and so somehow it was uplifting in the end. It wasn't a depressing thing for me."

Hunt Lieberson's diagnosis had been a wake-up call. "I turned my health around. My system had been run down for a few years. I had to stop in my tracks and make some changes." That involved moving from chilly Nova Scotia to Santa Fe in New Mexico, and a renewed resoluteness about rationing projects and performances.

Her caution about accepting engagements, however, is balanced by a passionate commitment to those projects she does take on. According to Clements: "She invests all these parts... with a whole extra dimension, turns them into thinking, feeling human beings, not just operatic cutouts. She can do that only because she rations what she does. It's to do with her focus."

Pickard says: "The choices she makes are not about building a career in the conventional sense but working on repertory she cares about with people she feels comfortable with."

She is less attracted to the recording studio than to live performance, and seems uninterested in being marketed in the glossy, trite way that so many of her peers are. She gives few interviews, has no real interest in "putting herself about" - and so remains, for now at least, one of the best-kept secrets in music.

Life in short

Born: 1954, San Francisco, California

Education: Berkeley high school; San Jose State University; Boston Conservatory

Family: Married the composer Peter Lieberson in 1999

Career:

· Began her musical career as a violist and supported herself through college as a freelance musician

· Started singing in her mid-20s and joined a professional choir after training at the Boston Conservatory as a lyric soprano

· Her breakthrough audition was in 1985 when she landed the role of Sesto in Peter Sellars' production of Julius Caesar

· Her performances include appearances with the Metropolitan Opera in New York, Opera National de Paris, the Los Angeles Philharmonic Orchestra, at Glyndebourne, the Boston Symphony Orchestra under André Previn, Wigmore Hall and the Edinburgh festival